OFDM receiving device and OFDM receiving method

ABSTRACT

The time required for switch the channel can be remarkably curtailed. When broadcasting signals through a plurality of information channels with an OFDM system, the plurality of information channels are multiplexed in the sense of frequency and collectively subjected to IFFT modulation for connected transmission instead of subjecting the plurality of information channels independently to OFDM modulation for transmission. With this arrangement, the efficiency of exploitation of frequencies is improved. According to the invention, the OFDM frames are synchronized for each information channel for the purpose of connected transmission. Then, the OFDM receiver can switch the information channel for signal reception, maintaining the frame synchronizing signals.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an OFDM receiving device and also to anOFDM receiving method to be used for digital broadcasting by orthogonalfrequency division multiplexing (OFDM).

[0003] 2. Related Background Art

[0004] Various modulation techniques referred as orthogonal frequencydivision multiplexing (OFDM) have been proposed in recent years forbroadcasting digital signals. With an OFDM system, a transmission bandis provided with a number of orthogonally arranged sub-carriers and dataare assigned to the amplitude and the phase of each sub-carrier for thepurpose of digital modulation by PSK (phase shift keying) or QAM(quadrature amplitude modulation).

[0005] With OFDM, while each sub-carrier has a small bandwidth becausethe transmission band is divided into a number of sub-carriers and hencethe modulation speed per sub-carrier is low, the overall transmissionrate remains practically same as that of any conventional modulationsystem. Additionally, OFDM is characterized by a low symbol rate alsodue to the fact that a number of sub-carriers are used in parallel forsignal transmission. Therefore, with OFDM, the time length of amulti-path can be reduced relative to that of a symbol in order toreduce the possible interference in the multi-path. Furthermore, withOFDM, since data are assigned to a number sub-carriers, thetransmission/reception circuit can be configured by using an IFFT(Inverse Fast Fourier Transform) operational circuit for modulation andan FFT (Fast Fourier Transform) operational circuit for demodulation.

[0006] Because of the above identified advantages of OFDM, studies havebeen made to apply it to ground wave digital broadcasting that can bestrongly affected by multi-path interference. In Japan, a standardreferred to as ISDB-T (Integrated Services DigitalBroadcasting-Terrestrial) have been proposed.

[0007] Meanwhile, with OFDM, each channel is normally provided with bandgaps for preventing interference from the adjacent channels, which arereferred to as guard bands and have a predetermined frequency bandwidthas shown in FIG. 1 of the accompanying drawing. However, the provisionof guard bands inevitably increases the bandwidth occupied by eachchannel to consequently reduce the efficiency of frequency exploitation.

[0008] In view of this problem, the applicant of the present patentapplication has proposed in International Publication No. WO 00/52861 aconnected transmission method (which is also referred to as concatenatedtransmission method) for OFDM signals, by which the center frequenciesof the OFDM signals in the frequency domains of a plurality ofinformation channels are modified respectively and then the OFDM signalsin the frequency domains of the plurality of information channels aremultiplexed in the sense of frequency and collectively subjected toinverse Fourier transform.

[0009] With the proposed connected transmission method for OFDM signals,when transmitting three streams of information through respective threeinformation channels (Ch1, Ch2, Ch3), the guard bands separating thechannels can be removed and the three information channels can beconnected in the sense of frequency axis for signal transmission asillustrated in FIG. 2.

[0010] An OFDM transmitter adapted to connected transmission of OFDMsignals will be described below in greater detail.

[0011]FIG. 3 is a schematic block diagram of an OFDM transmitter adaptedto connected transmission of OFDM signals. While any number of channelscan be connected together with the proposed technique, it is assumedhere that three channels (Ch1, Ch2, Ch3) are connected to transmit threestreams of information. Also assume that the center frequencies of theinformation channels in the RF band are respectively f₁ for the firstchannel, f₂ for the second channel and f₃ for the third channel as shownin FIG. 2.

[0012] The OFDM transmitter 101 comprises a first channel encoder 102-1,a second channel encoder 102-2, a third channel encoder 102-3, a firstfrequency converter section 103-1, a second frequency converter 103-2, athird frequency converter 103-3, a multiplexer 104, an IFFT operationalcircuit 105, a guard interval adder 106, an orthogonal modulator 107, afrequency converter 108 and an antenna 109.

[0013] The first channel encoder 102-1 receives an information stream asinput through the first information channel. It is adapted to operatefor Reed-Solomon coding, energy dispersion, interleaving, convolutionalcoding, mapping and configuring an OFDM frame. The first channel encoder102-1 generates first channel data as an OFDM signal of the frequencydomain of the first channel by carrying out the above operations. Thecenter frequency of the first channel data generated as an OFDM signalof the frequency domain output from the first channel is made equal to0.

[0014] The second channel encoder 102-2 and the third channel encoder102-3 operate like the first channel encoder 102-1 respectively for theinformation stream of the second information channel and that of thethird information channel. Additionally, the center frequency of theOFDM signal of the frequency domain output from the second channel (thesecond channel data) and that of the OFDM signal of the frequency domainoutput from the third channel (the third channel data) are also madeequal to 0.

[0015] The first frequency converter 103-1 performs a processingoperation of frequency conversion for shifting the center frequency ofthe first channel data (the OFDM signal of the corresponding frequencydomain) output from the first channel encoder 102-1. More specifically,the first frequency converter 103-1 converts the center frequency of thefirst channel data from 0 to (f₁-f₂).

[0016] The second frequency converter 103-2 performs a processingoperation of frequency conversion for shifting the center frequency ofthe second channel data (the OFDM signal of the corresponding frequencydomain) output from the second channel encoder 102-2. More specifically,the second frequency converter 103-2 converts the center frequency ofthe second channel data from 0 to (f₂-f₂).

[0017] The third frequency converter 103-3 performs a processingoperation of frequency conversion for shifting the center frequency ofthe third channel data (the OFDM signal of the corresponding frequencydomain) output from the third channel encoder 102-3. More specifically,the third frequency converter 103-3 converts the center frequency of thethird channel data from 0 to (f₃-f₂).

[0018] It will be appreciated that the second channel data is actuallynot subjected to frequency conversion because it is located at thecenter of the three channels connected for data transmission.

[0019] The multiplexer 104 multiplexes the channel data output from thefirst frequency converter 103-1, the second frequency converter 103-2and the third frequency converter 103-3 in the sense of frequency togenerate a multiplexed signal.

[0020] The IFFT operational circuit 105 performs an operation of inverseFourier transform collectively on the multiplexed signals of the threechannel data as multiplexed by the multiplexer 104 to generate an OFDMsignal of the base band of time domain. As shown in FIG. 4, thefrequency characteristics of the generated OFDM signal of the base bandare such that the center frequency of the first information channel is(f₁-f₂), that of the second information channel is 0 and that of thethird information channel is (f₃-f₂). In the OFDM signal of the baseband, the pieces of information of the first through third informationchannels are subjected to frequency division and multiplexing andmaintain orthogonality in order to eliminate any inter-code interferenceamong all the carrier waves.

[0021] The guard interval adder 106 adds a guard interval to the OFDMsignal of the base band from the IFFT operational circuit 105. As shownin FIG. 5, each signal to be transmitted by the OFDM system is actuallytransmitted on the basis of a unit of symbol referred to as OFDM symbol.An OFDM symbol comprises an effective symbol representing a signalperiod during which an IFFT operation is performed for transmission anda guard interval where a rear part of the effective symbol is copied.The guard interval is arranged in a front part of the OFDM symbol. Theguard interval adder 106 generates such a guard interval and adds it tothe effective symbol.

[0022] The orthogonal modulator 107 orthogonally modulates the OFDMsignal of the base band, to which a guard interval is added, relative tothe carrier wave with an intermediate frequency band of frequency f_(IF)and outputs an IF signal.

[0023] The frequency converter 108 multiplies the IF signal output fromthe orthogonal modulator 107 by the carrier wave signal with a frequencyof f₂+f_(IF) to produce a signal to be transmitted in an RF signal band.

[0024] The signal produced by the frequency converter 108 is thentransmitted by way of the antenna 109.

[0025] Thus, as described above, the OFDM transmitter can carry out aconnected transmission of OFDM signals by changing the centerfrequencies of the channel data of the three information channels (theOFDM signals of the frequency domains), multiplexing them in the senseof frequency and performing an operation of inverse Fourier transformcollectively on the OFDM signals of the frequency domains of theinformation channels.

[0026] With such a connected transmission, a single operation of IFFT isperformed collectively on the three channels to maintain orthogonalityin order to eliminate any inter-code interference among thesub-carriers. As a result, no interference occurs in the connected threechannels and therefore the OFDM transmitter 101 can transmit informationfor three channels without providing guard bands for preventinginterferences with adjacent channels.

[0027] An OFDM receiver for receiving such a signal is adapted to detectthe IF signal by tuning the oscillation frequency of the localoscillator in the center frequency of the desired information channel.For instance, the oscillation frequency of the local oscillator will betuned in the frequency (f₁) for receiving the signal of the firstinformation channel, in the frequency (f₂) for receiving the signal ofthe second information channel and in frequency (f₃) for receiving thesignal of the third information channel. The detected IF signal is thenorthogonally demodulated by means of the carrier wave of the frequency(f_(IF)) and transformed into an OFDM signal of the base band of timedomain. It will be appreciated that the center frequency of the OFDMsignal of the base band is equal to 0 regardless of the informationchannel selected for signal reception. Then, the OFDM signal of the baseband is subjected to an operation of FFT to obtain the channel data ofthe OFDM signal of the frequency domain by demodulation.

[0028] Thus, if the OFDM signals of the frequency domains of a pluralityof information channels are multiplexed in the sense of frequency andsubjected collectively to an operation of inverse Fourier transform forconnected transmission, the OFDM receiver can selectively receive thesignal of only one of the channels by tunning the oscillation frequencyof the local oscillator in the center frequency of the desiredinformation channel.

[0029] Now, the frame configurations as defined in the ISDB-T Standard(in the case of Mode 1) for a broadcasting mode of digital ground wavebroadcasting will be discussed below.

[0030] As shown in FIGS. 6 and 7, according to the ISDB-T Standarddefines the data structure of an OFDM frame for data to be transmitted.FIG. 6 illustrates the frame configuration to be used for modulating aninformation signal by differential modulation (differential quadraturephase shift keying—DQPSK) and FIG. 7 shows the frame configuration to beused for modulating an information signal by synchronous modulation(quadrature phase shift keying—QPSK, 16 quadrature amplitudemodulation—16QAM, 64 quadrature amplitude modulation—64QAM).

[0031] Referring to FIGS. 6 and 7, a total of 108 data (with carriernumbers #0 through #107) are transmitted by a symbol. The unit of dataof a symbol is referred to as OFDM symbol. Also note that 204 OFDMsymbols (with symbol numbers #0 through #203) constitute an OFDM frame.

[0032] An OFDM frame contains information signals (S_(0,0) throughS_(95,203)) that are orthogonally modulated by QPSK, 16QAM or 64QAMalong with various control signals such as CP (Continual Pilot) signal,TMCC (Transmission and Multiplexing Configuration Control) signal, AC(Auxiliary Channel) signal and SP (Scattered Pilot) signal.

[0033] The CP signal is a signal with a fixed phase and a fixedamplitude. When modulating the information signal by differentialmodulation, a CP signal is arranged at the leading carrier of each OFDMsymbol (at a position where the frequency is lowest). When transmittingthe information signal by connected transmission, a CP signal isarranged at the rightmost position of the connected transmission band(at a position where the frequency is highest).

[0034] The SP signal is a signal modulated by BPSK and, as shown in FIG.7, arranged in such a way that it is inserted once in every 12 carriersin the sense of frequency and once in every 4 symbols in the sense ofsymbol. The SP signal is used to estimate the characteristics of thetransmission path when the receiver side equalizes the waveform.Therefore it is inserted only for synchronous modulation involvingwaveform equalization (QPSK, 16QAM, 64QAM).

[0035] The TMCC signal and the AC signal are signals modulated also byBPSK and arranged at respective positions in each symbol as shown inFIGS. 8 and 9. FIG. 8 shows their positions in an OFDM frame fordifferential modulation and FIG. 9 shows their positions in an OFDMframe for synchronous modulation. The AC signal is used for thetransmission of additional information, while the TMCC signal is usedfor the transmission of transmission control information.

[0036] The TMCC signal carries 204-bit (B₀ through B₂₀₃) informationthat is completely contained in a unit of OFDM frame. FIG. 10 shows thecontents of information assigned to a TMCC signal.

[0037] Bit B₀ is a bit to which the signal operating as reference foramplitude and phase of differential modulation is assigned.

[0038] Bits B₁ through B₁₆ are bits to which a sync code (synchronizingsignal) to be inverted on a frame by frame basis is assigned. Thereceiver detects the bit pattern of the sync code to detect thesynchronization of the TMCC signal and that of the OFDM frame.

[0039] Bits B₁₇ through B₁₉ are bits to which the identification signalof the segment for identifying the frame as one for synchronousmodulation or for differential modulation is assigned.

[0040] Bits B₂₀ through B₁₂₁ are bits to which TMCC information (of 120bits) is assigned. The TMCC information describes the carrier modulationmode of the information signal, the convolutional coding ratio, theinterleave length, the number of segments and so on.

[0041] Bits B₁₂₂ through B₂₀₃ are bits to which parity bits areassigned.

[0042] The transmitter generates the OFDM frame in the frame configuringsection of the channel encoder. The receiver firstly establishes thesynchronism of the symbols on a symbol by symbol basis and performs anoperation of FFT. Subsequently, it detects the synchronizing signaldescribed in the TMCC signal and establishes the synchronism of theframes to decode the data contained therein.

[0043] When the receiver shifts from a channel to another, it selectsthe oscillation frequency of the local oscillator for another time torestart the operation of receiving an RF signal. Therefore, whenshifting the channel, the receiver has to carry out for another time theoperation of establishing the synchronism of the symbols on a symbol bysymbol basis, performing an operation of FFT, subsequently detecting thesync code described in the TMCC signal and then establishing thesynchronism of the frames to decode the data contained therein.

[0044] However, when detecting the synchronism of frames, it isnecessary to detect the sync codes of at least two frames. This meansthat a time span longer than that of a frame period has to be spent forthe detection. For instance, according to the ISDB-T Standard, the framelength of an OFDM frame is about 250 ms at most. Then, about 250 ms hasto be spent to detect the synchronism of frames. If the synchronism offrames is not detected, it is no longer possible to extract the SPsignal that defines the positional arrangement of frames and data on aframe by frame basis, decode the TMCC signal and identify the switchingposition of puncturing. Then, no data can be output. In short,conventionally a very large amount of time has to be spent for aswitching operation from the time of a channel shift to the that ofoutputting audio and video data after the shift.

[0045] This problem arises regardless if connected transmission is usedor not.

BRIEF SUMMARY OF THE INVENTION

[0046] In view of the above identified circumstances, it is thereforethe object of the present invention to provide an OFDM receiving deviceand an OFDM receiving method that can reduce the time necessary forswitching the information channel to be used for reception.

[0047] According to the present invention, the above object is achievedby providing An OFDM receiving device for receiving an orthogonallyfrequency divided and multiplexed signals comprising: a receiver forselecting an information channel to be received and receiving the RFsignal of the selected information channel; an orthogonal demodulatorfor orthogonally demodulating the signal received by said receiver andoutputting an OFDM signal of a base band; a Fourier transform sectionfor performing an operation of Fourier transform on said OFDM signal ofthe base band and outputting an OFDM signal of a frequency domain; adecoder for decoding said OFDM signal of the frequency domain; and aframe synchronism control section for detecting the synchronism oftransmission frames of said channel data and controlling the synchronismof said decoder; said frame synchronism control section being adapted tomaintain the synchronism of the transmission frames for switching theinformation channel to be used for signal reception among the channelsconnected for transmission in the case of receiving OFDM signals offrequency domains of a plurality of information channels multiplexed ina direction of frequency and collectively subjected to an operation ofinverse Fourier transform for connected transmission.

[0048] With an OFDM receiving device according to the invention andhaving the above configuration, when the OFDM signals of frequencydomains of a plurality of information channels are multiplexed andcollectively subjected to an operation of inverse Fourier transform forconnected transmission and the information channel is switched forsignal reception among the channels connected from transmission, thesynchronism of the transmission frames is maintained. In other words,with the OFDM receiving device, the synchronism of the transmissionframes is not broken when switching the information channel for signalreception.

[0049] In another aspect of the invention, there is provided an OFDMreceiving device for receiving an orthogonally frequency divided andmultiplexed comprising: an OFDM receiving device comprising a receiverfor selecting an information channel to be received and receiving an RFsignal of the selected information channel; an orthogonal demodulatorfor orthogonally demodulating the signal received by said receiver andoutputting the OFDM signal of a base band; a Fourier transform sectionfor performing an operation of Fourier transform on said OFDM signal ofthe base band and outputting an OFDM signal of a frequency domain; adecoder for decoding said OFDM signal of the frequency domain; a controlsection for controlling said Fourier transform section and said decoder;and said OFDM signal of the base band containing information onconnected transmission indicating whether the OFDM signal to betransmitted to an information channel and the OFDM signal to betransmitted to other information channels are transmitted in a connectedstate by multiplexing the OFDM signals of the frequency domain of aplurality of the information channel in a direction of the frequency andperforming an inverse Fourier transform on the multiplexed OFDM signalscollectively, said control section being adapted to determine whetherthe information channel being used for signal reception is coupled tothe information channel to be selected for signal reception by switchingby referring to said information on connected transmission.

[0050] With an OFDM receiving device according to the invention andhaving the above configuration, when the information channel being usedfor signal reception is switched to some other information channel, itis determined if the information channel being used for signal receptionand the information channel to be selected for signal reception arecoupled for transmission or not as the OFDM signals of frequency domainsof a plurality of information channels are multiplexed and collectivelysubjected to an operation of inverse Fourier transform for coupledtransmission.

[0051] In still another aspect of the invention, there is also providedan OFDM receiving method for receiving orthogonally frequency dividedand multiplexed signals comprising a step of maintaining the synchronismof the transmission frames for switching the information channel to beused for signal reception among the channels connected for transmissionin the case of receiving OFDM signals of frequency domains of aplurality of information channels multiplexed in a direction offrequency and collectively subjected to an operation of inverse Fouriertransform for connected transmission.

[0052] With the above described OFDM receiving method, when the OFDMsignals of frequency domains of a plurality of information channels aremultiplexed and collectively subjected to an operation of inverseFourier transform for coupled transmission and the information channelconnected for transmission is switched among the plurality ofinformation channels, the switching operation is conducted whilemaintaining the synchronism of the transmission frames. In other words,with the OFDM receiving method, the synchronism of the transmissionframes is not broken when switching the information channel for signalreception.

[0053] In still another aspect of the invention, there is also providedan OFDM receiving method for receiving orthogonally frequency dividedand multiplexed signals comprising the steps of; receiving the OFDMsignal of the base band containing information on connected transmissionindicating whether the OFDM signal to be transmitted to an informationchannel and the OFDM signal to be transmitted to other informationchannels are transmitted in a connected state by multiplexing the OFDMsignals of the frequency domain of a plurality of the informationchannels in a direction of the frequency and performing an inverseFourier transform on the multiplexed OFDM signals collectively; and ofdetermining whether the information channel being used for signalreception is coupled to the information channel to be selected forsignal reception by switching being determined by referring to theinformation on the connected transmission.

[0054] With the above described OFDM receiving method, the existence ornon-existence of a coupled state of the information channel being usedfor signal reception and the information channel to be selected forsignal reception by switching is determined by referring to theinformation on connected transmission.

[0055] Thus, with an OFDM receiving device and an OFDM receiving methodaccording to the invention, when the OFDM signals of frequency domainsof a plurality of information channels are multiplexed and collectivelysubjected to an operation of inverse Fourier transform for connectedtransmission and the information channel is switched for signalreception among the channels connected from transmission, thesynchronism of the transmission frames is maintained. In other words,with the OFDM receiving device, the synchronism of the transmissionframes is not broken when switching the information channel for signalreception.

[0056] With this arrangement according to the invention, the timerequired for switching the information channel to be used for signalreception can be reduced so that the operation of reproducing video andaudio signal and outputting data can be started very quickly.

[0057] Additionally, with an OFDM receiving device and an OFDM receivingmethod according to the invention and having the above configuration,when the information channel being used for signal reception is switchedto some other information channel, it is determined if the informationchannel being used for signal reception and the information channel tobe selected for signal reception are coupled for transmission or not asthe OFDM signals of frequency domains of a plurality of informationchannels are multiplexed and collectively subjected to an operation ofinverse Fourier transform for coupled transmission.

[0058] With this arrangement according to the invention, if the newlyselected information channel is coupled for transmission or not can bedetermined with ease.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0059]FIG. 1 is a schematic illustration of guard bands arranged betweeninformation channels;

[0060]FIG. 2 is a schematic illustration of signals for connectedtransmission;

[0061]FIG. 3 is a schematic block diagram of a conventional OFDMtransmitter;

[0062]FIG. 4 is a schematic illustration of the frequencycharacteristics of OFDM signals of base bands obtained by performing anoperation of IFFT collectively for three channels;

[0063]FIG. 5 is a schematic illustration of OFDM symbols containingguard intervals;

[0064]FIG. 6 is a schematic illustration of the frame configuration thatcan be used when modulating an information signal by differentialmodulation (DQPSK);

[0065]FIG. 7 is a schematic illustration of the frame configuration thatcan be used when modulating an information signal by synchronousmodulation (QPSK, 16QAM, 64QAM);

[0066]FIG. 8 is a schematic illustration of the positional arrangementof TMCC signals and AC signals in an OFDM frame for differentialmodulation;

[0067]FIG. 9 is a schematic illustration of the positional arrangementof TMCC signals and AC signal in an OFDM frame for synchronousmodulation;

[0068]FIG. 10 is a schematic illustration of the contents of informationcontained in a TMCC signal;

[0069]FIG. 11 is a schematic block diagram illustrating source encodersand an OFDM transmitter;

[0070]FIG. 12 is a schematic block diagram of an embodiment of OFDMtransmitter according to the invention;

[0071]FIG. 13 is a schematic block diagram of the frequency converter ofthe embodiment of FIG. 12;

[0072]FIG. 14 is a schematic illustration of the frame synchronizationof multiplexed OFDM signals;

[0073]FIG. 15 is a schematic block diagram illustrating an OFDMtransmitter and an MPEG decoder;

[0074]FIG. 16 is a schematic block diagram of an embodiment of OFDMreceiver according to the invention;

[0075]FIG. 17 is a schematic illustration of the number of connectedsegments and the segment numbers of the signals to be transmitted asdescribed in TMCC information;

[0076]FIG. 18 is a schematic illustration of a specific description on anumber of connected segments;

[0077]FIG. 19 is a schematic illustration of connected transmission of13 segments;

[0078]FIG. 20 is a schematic illustration of connected transmission of 3segments;

[0079]FIG. 21 is a schematic illustration of connected transmission of 6segments;

[0080]FIG. 22 is a schematic illustration of a specific description onsegment numbers;

[0081]FIG. 23 is a schematic illustration of a shift to a channel insidea connected transmission scheme and to a channel outside the scheme;

[0082]FIG. 24 is a schematic illustration of the ID of a group ofsegments for connected transmission; and

[0083]FIG. 25 is a schematic illustration of a transmission of fivegroups, each containing two segments, for connected transmission andthat of a single segment for isolated transmission.

DETAILED DESCRIPTION OF THE INVENTION

[0084] Now, the present invention will be described by referring toviews of the accompanying drawing that illustrate an embodiment of OFDMtransmitter and an embodiment of OFDM receiver according to theinvention.

[0085] Firstly, the configuration of the OFDM transmitter will bedescribed.

[0086] Referring to FIG. 11, the transmission side comprises a pluralityof source encoders 1 a (1 a-1 through 1 a-n) and an OFDM transmitter 1.The source encoders 1 a are adapted to receive video data and audio dataof a plurality of base bands, which data are then subjected tocompression coding according to the MPEG-2 Systems to generate aplurality of program streams. The source encoders 1 a are also adaptedto multiplex the plurality of program streams into transport streams asdefined in the MPEG-2 Systems. The OFDM transmitter 1 multiplexes theplurality of transport streams output from the plurality of sourceencoders 1 a for connected transmission.

[0087]FIG. 12 is a block diagram of an embodiment of OFDM transmitteraccording to the invention.

[0088] Referring to FIG. 12, the OFDM transmitter is adapted to combinethree channels for connected transmission as in the case of the abovedescribed conventional OFDM transmitter. Assume that the centerfrequencies of the information channels in an RF band are f₁ for thefirst information channel, f₂ for the second information channel and f₃for the third information channel as shown in FIG. 2 and describedearlier.

[0089] The OFDM transmitter 1 comprises a first channel encoder 2-1, asecond channel encoder 2-2, a third channel encoder 2-3, a synchronismcontrol section 3, a first frequency converter 4-1, a second frequencyconverter 4-2, a third frequency converter 4-3, a multiplexer 5, an IFFToperating circuit 6, an guard interval adder 7, an orthogonal modulator8, a frequency converter 9 and an antenna 10.

[0090] The first channel encoder 2-1 receives an information stream asinput. The first channel encoder 2-1 is adapted to perform processingoperations of Reed-Solomon coding, energy dispersion, interleaving,convolutional coding, mapping, OFDM frame configuring and so on. Thefirst channel encoder 2-1 is provided with a frame configuring section2-1 a for configuration the OFDM frame. The frame configuring section2-1 a is adapted to add a CP signal, an AC signal, a TMCC signal and anSP signal to the coded information signal to configure an OFDM framecontaining 204 OFDM symbols as shown in FIGS. 6 and 7. The synchronoustiming of the operation of the frame configuring section 2-1 ofconfiguring the OFDM frame is controlled by the synchronism controlsection 3. More specifically, the symbol and the timing of cutting outthe frame is controlled by the synchronism control section 3. The firstchannel encoder 2-1 performs the above processing operation to generatefirst channel data as OFDM signal of a frequency domain. The centerfrequency of the first channel data, or the OFDM signal of the frequencydomain, is made equal to 0.

[0091] The second channel encoder 2-2 and the third channel encoder 2-3operate just like the first channel encoder 2-1 respectively for theinformation stream of the second information channel and that of thethird information channel. Similarly, the second channel encoder 2-2 andthe third channel encoder 2-3 are provided with respective frameconfiguring sections 2-2 a and 2-3 a for configuration the respectiveOFDM frames. Again, the synchronous timing of the operation of the frameconfiguring section 2-2 a and that of the frame configuring section 2-3a of configuring the respective OFDM frames are controlled by thesynchronism control section 3. The center frequencies of the OFDMsignals of the frequency domains of the second channel encoder 2-2 andthe third channel encoder 2-3 (the second channel data and the thirdchannel data) are also made equal to 0.

[0092] The synchronism control section 3 controls the synchronous timingof the OFDM frame for the first channel encoder 2-1, that of the OFDMframe for the second channel encoder 2-2 and that of the OFDM frame forthe third channel encoder 2-3. In other words, the synchronism controlsection 3 controls the synchronism of the frames in such a way that allthe OFDM frames of the first through third channel data temporally agreewith each other. More specifically, the synchronous timing of each ofthe OFDM frames is so controlled that the timing of the leading OFDMsymbol (#0) of each of the OFDMs is made to agree with those of theleading OFDM symbols of the remaining channels.

[0093] The first frequency converter 4-1 performs an operation offrequency conversion to shift the center frequency of the first channeldata (the OFDM signal of the frequency domain) output from the firstchannel encoder 2-1. More specifically, the first frequency converter4-1 shifts the center frequency of the first channel data from 0 to(f₁-f₂).

[0094] The second frequency converter 4-2 performs an operation offrequency conversion to shift the center frequency of the second channeldata (the OFDM signal of the frequency domain) output from the secondchannel encoder 2-2. More specifically, the second frequency converter4-2 shifts the center frequency of the second channel data from 0 to(f₂-f₂).

[0095] The third frequency converter 4-3 performs an operation offrequency conversion to shift the center frequency of the third channeldata (the OFDM signal of the frequency domain) output from the thirdchannel encoder 2-3. More specifically, the third frequency converter4-3 shifts the center frequency of the third channel data from 0 to(f₃-f₂).

[0096]FIG. 13 shows a circuit diagram of the first frequency converter4-1, the second frequency converter 4-2 and the third frequencyconverter 4-3.

[0097] The frequency conversion circuit comprises a phase shifter 11, aphase angle generator 12 and an accumulator 13.

[0098] The phase shifter 11 receives as input a complex signal mappedaccording to a given modulation system such as BPSK, DQPSK, QPSK, 16QAMor 64QAM. The signal point of the input complex signal is expressed by(I, Q). The phase angle generator 12 receives as input the quantity offrequency shift Δf and the length of guard interval ΔT. The quantity offrequency shift Δf represents the difference between the centerfrequency of the RF frequency band of each information channel and thecenter frequency of the RF frequency band of the multiplexed signal forconnected transmission. Therefore, the quantity of frequency shift Δf ofthe first information channel is equal to (f₁-f₂) and that of frequencyshift between the Δf of the second information channel is equal to(f₂-f₂), while that of frequency shift Δf of the third informationchannel is equal to (f₃-f₂).

[0099] The phase angle generator 12 generates a phase angle θ by usingformula (1) below;

θ=f(Δf, ΔT)=2πΔf(T+ΔT)  (1)

[0100] where T is the effective symbol period of the OFDM signal of thebase band.

[0101] The phase angle generated by the phase angle generator 12 isinput to the accumulator 12.

[0102] The accumulator 12 accumulates the input phase angle θ for eachsymbol and outputs the accumulated value θ′. The accumulated value θ′ isthen input to the phase shifter 11.

[0103] The phase shifter 11 substitutes the accumulated value θ′ forformula (2) below to shift the frequency for the signal point (I, Q).$\begin{matrix}{\begin{pmatrix}{I_{out}(n)} \\{Q_{out}(n)}\end{pmatrix} = {\begin{pmatrix}{\cos \quad \theta_{{clk}{(n)}}} & {{- \sin}\quad \theta_{{clk}{(n)}}} \\{\sin \quad \theta_{{clk}{(n)}}} & {\cos \quad \theta_{{clk}{(n)}}}\end{pmatrix}\begin{pmatrix}{I_{in}(n)} \\{Q_{in}(n)}\end{pmatrix}}} & (2)\end{matrix}$

[0104] The first frequency converter 4-1, the second frequency converter4-2 and the third frequency converter 4-3 outputs the obtained signalpoint (I′, Q′) to the multiplexer 5.

[0105] It will be appreciated that the second channel data is actuallynot subjected to frequency conversion because it is located at thecenter of the three channels connected for data transmission.

[0106] The multiplexer 5 multiplexes the channel data output from thefirst frequency converter 4-1, the second frequency converter 4-2 andthe third frequency converter 4-3 in the sense of frequency to generatea multiplexed signal. The multiplexed signal obtained by themultiplexing contains the first information channel, the secondinformation channel and the third information channel in the sense offrequency and synchronized for the frames on time base.

[0107] The IFFT operational circuit 6 performs an operation of inverseFourier transform collectively on the multiplexed signals of the threechannel data as multiplexed by the multiplexer 5 to generate an OFDMsignal of the base band of time domain. As shown in FIG. 4, thefrequency characteristics of the generated OFDM signal of the base bandare such that the center frequency of the first information channel is(f₁-f₂), that of the second information channel is 0 and that of thethird information channel is (f₃-f₂). In the OFDM signal of the baseband, the pieces of information of the first through third informationchannels are subjected to frequency division and multiplexing andmaintain orthogonality in order to eliminate any inter-code interferenceamong all the carrier waves.

[0108] The guard interval adder 7 adds a guard interval to the OFDMsignal of the base band from the IFFT operational circuit 6.

[0109] The orthogonal modulator 8 orthogonally modulates the OFDM signalof the base band, to which a guard interval is added, relative to thecarrier wave with an intermediate frequency band of frequency f_(IF) andoutputs an IF signal.

[0110] The frequency converter 9 multiplies the IF signal output fromthe orthogonal modulator 8 by the carrier wave signal with a frequencyof f₂+f_(IF) to produce a signal to be transmitted in an RF signal band.

[0111] The signal produced by the frequency converter 9 is thentransmitted by way of the antenna 10.

[0112] Thus, as described above, the OFDM transmitter 1 can carry out aconnected transmission of OFDM signals by changing the centerfrequencies of the channel data of the three information channels (theOFDM signals of the frequency domains), multiplexing them in the senseof frequency and performing an operation of inverse Fourier transformcollectively on the OFDM signals of the frequency domains of theinformation channels.

[0113] With such a connected transmission, a single operation of IFFT isperformed collectively on the three channels to maintain orthogonalityin order to eliminate any inter-code interference among thesub-carriers. As a result, no interference occurs in the connectedthrough channels and therefore the OFDM transmitter 101 can transmitinformation for three channels without providing guard bands forpreventing interferences with adjacent channels.

[0114] Additionally, the OFDM transmitter 1 synchronizes the OFDM framesof the plurality of information channel that are connected fortransmission.

[0115] Now, the configuration of the reception side will be described.

[0116] Referring to FIG. 15, the reception side comprises an OFDMreceiver 20 and an MPEG decoder 21. The OFDM receiver 20 is adapted toreceive the broadcast wave transmitted from the OFDM transmitter 1 anddemodulates the transport streams of the MPEG-2 Systems. The MPEGdecoder 21 selects an appropriate program stream out of the demodulatedtransport streams and MPEG-decodes it in order to output video data andaudio data.

[0117]FIG. 16 is a schematic block diagram of an embodiment of OFDMreceiver according to the invention.

[0118] Referring to FIG. 16, the OFDM receiver 20 comprises an antenna22, a tuner 23, a bandpass filter (BPF) 24, an A/D converter 25, adigital orthogonal demodulator 26, an fc correction circuit 27, an FFToperational circuit 28, a narrow band fc error computing window sync(FAFC W-Sync) circuit 29, a broad band fc error computing (WAFC) circuit30, a numerical value control oscillator (NCO) 31, an equalizer 32, afrequency-directional de-interleaver 33, a time-directionalde-interleaver 34, a de-mapping circuit 35, an error correction circuit36, a TMCC demodulator 37, a control section 38 and a memory 39.

[0119] The broadcast wave transmitted from said OFDM transmitter 1 isreceived by way of the antenna 22 of the OFDM receiver 20 and fed to thetuner 23 as an RF signal.

[0120] The RF signal received by the antenna 22 is subjected tofrequency conversion by the tuner 23 typically comprising a localoscillator and a multiplier to produce an IF signal, which IF signal isthen fed to the BPF 4. The local oscillation frequency of the tuner 23is selected by the control section 38 so as to correspond to the channelselected by the user. For example, if the first information channel(CH1) is used for signal reception, the local oscillation frequency istuned to (f₁). If the second information channel (CH2) is used forsignal reception, the local oscillation frequency is tuned to (f₂).Similarly, if the third information channel (CH3) is used for signalreception, the local oscillation frequency is tuned to (f₃). The IFsignal output from the tuner 23 filtered by the BPF4 and subsequentlydigitized by the A/D converter 25 before fed to the digital orthogonaldemodulator 26.

[0121] The digital orthogonal demodulator 26 orthogonally demodulatesthe digitized IF signal by means of a carrier signal with apredetermined frequency (f_(c): carrier frequency) and outputs the OFDMsignal of the base band. The OFDM signal of the base band output fromthe digital orthogonal demodulator 26 is a signal of so-called timedomain before being subjected to an FFT operation. As the OFDM signal ofthe base band of time domain is orthogonally demodulated, an complexsignal having a real axis component (I channel signal) and an imaginaryaxis component (Q channel signal). The OFDM signal of the base bandoutput from the digital orthogonal demodulator 26 is then fed to the fccorrection circuit 27.

[0122] The fc correction circuit 27 performs a complex multiplication ofthe fc error correction signal output from the NCO 31 and the base bandOFDM signal to correct the carrier frequency error of the base band OFDMsignal. The carrier frequency error is the positional error of thecenter frequency of the base band OFDM signal typically produced by thedisplacement of the reference frequency output from the localoscillator. The error rate of the output data increases as this errorbecomes significantly large. The base band OFDM signal corrected for thecarrier frequency error by the fc correction circuit 27 is then fed tothe FFT operational circuit 28 and the FAFC•W-Sync circuit 29.

[0123] The FFT operational circuit 28 performs an FFT operation on thebase band OFDM signal and extracts and outputs the data orthogonallymodulated relative to each sub-carrier. The signal output from the FFToperational circuit 28 is a signal of so-called a frequency domain thathas been subjected to an FFT operation.

[0124] The FFT operational circuit 28 takes out a signal within theeffective range of symbol length (e.g., of 256 samples) from a singleOFDM symbol to remove the guard interval from the single OFDM symbol andperforms an FFT operation on the taken out base band OFDM signal. Morespecifically, the position for starting the FFT operation is anywherebetween the boundary of the OFDM symbol to the end position of the guardinterval. This range of operation is referred to as FFT window.

[0125] The OFDM signal of the frequency domain output from the FFToperational circuit 28 is a complex signal having a real axis component(I channel signal) and an imaginary axis component (Q channel signal)like the base band OFDM signal of time domain. The OFDM signal of thefrequency domain is then fed to the WAFC circuit 30 and the equalizer32.

[0126] The FAFC•W-Sync circuit 29 and the WAFC circuit 30 compute thecarrier frequency errors contained in the output signal of the fccorrection circuit 27. More specifically, the FAFC•W-Sync circuit 29 isresponsible for computing the narrow band fc error with an accuracylevel of ±½ or less of the frequency gap of sub-carriers and the WAFCcircuit 30 computes the broad band fc error with an accuracy level ofthe frequency gap of sub-carriers. The carrier frequency errors asdetermined by the FAFC circuit 29 and the WAFC circuit 30 are then fedto the NCO 31.

[0127] The FAFC•W-Sync circuit 29 also determines the timing of startingthe FFT operation of the FFT operational circuit 28 and control thescope of the FFT operation (FFT window). The operation of controllingthe FFT window is conducted on the basis of the information on theboundary position of the OFDM symbol obtained when computing the narrowband carrier frequency error with the accuracy level of ±½ or less ofthe frequency gap of sub-carriers and the length of the guard intervalof the OFDM signal. The ISDB-T Standard defines four patterns for thelength of guard interval. They are ¼, ⅛, {fraction (1/16)} and {fraction(1/32)} in terms of the ratio to the length of the effective symbol. Thelength of the guard interval of the received OFDM signal is selected bythe control section 38.

[0128] The NCO 31 adds the narrow band carrier frequency error with theaccuracy level of ±½ or less of the frequency gap of sub-carriers ascomputed by the FAFC circuit 29 and the broad band fc error with theaccuracy level of the frequency gap of sub-carriers as computed by theWAFC circuit 30 and outputs the fc error correction signal obtained byadding them, the frequency of which fc error correction signal increasesor decreases as a function of the carrier frequency error obtained bythe addition. The fc error correction signal is a complex signal to befed to the fc correction circuit 27. The fc error correction signal isthen subjected to a complex multiplication with the base band OFDMsignal by the fc correction circuit 27 to remove the carrier frequencyerror component of the base band OFDM signal.

[0129] The equalizer 32 equalizes the phase and the amplitude of theOFDM signal of the frequency domain typically by using a scattered pilotsignal (SP signal). The OFDM signal of the frequency domain equalizedfor the phase and the amplitude is then fed to the frequency-directionalde-interleaver 33 and the TMCC decoder 37. If the transmitted signal isa signal that is subjected to differential modulation (DQPSK), theoperation of the equalizer 32 is not required.

[0130] The frequency-directional de-interleaver 33 de-interleaves thedata that is interleaved in the sense of frequency at the transmitterside according to the interleaving pattern of the signal. Thefrequency-directional de-interleaved data is then fed to thetime-directional de-interleaver 34.

[0131] The time-directional de-interleaver 34 de-interleaves the datainterleaved in the sense of time at the transmitter side according tothe interleaving pattern of the signal. The ISDB-T Standard defines fiveinterleaving patterns for each mode. For instance, five patterns thatmake the numbers of delay correction symbols equal to 0, 28, 56, 112 and224 respectively are defined. The interleaving pattern to be used forthe de-interleaving operation is selected by the control section 38. Thedata that is de-interleaved in the sense of time is then fed to thede-mapping circuit 35.

[0132] The de-mapping circuit 35 performs a de-mapping operationaccording to a predetermined carrier modulation system and demodulatesthe data that ore orthogonally modulated in the respective sub-carriersof the OFDM signal of the frequency domain. The ISDB-T Standard definesthe demodulation systems of DQPSK, QPSK, 16QAM and 64 QAM. The mappingpattern required for the de-mapping operation of the de-mapping circuit35 is selected by the control section 38. The data demodulated by thede-mapping circuit 35 is then fed to the error correction circuit 36.

[0133] The error correction circuit 36 performs a Viterbi decodingoperation on the data encoded by using punctured convolutional codes atthe transmitter side and an error correcting operation by using theReed-Solomon codes added thereto as external codes. The ISDB-T Standarddefines the coding ratios of punctured convolutional codes of ½, ⅔, ¾, ⅚and ⅞. the coding ratio of punctured convolutional codes to be used forthe Viterbi decoding is selected by the control section 38.

[0134] The data corrected for errors by the error correction circuit 36is then typically fed to an MPEG decoder arranged downstream.

[0135] The TMCC decoder 37 extracts the TMCC signal inserted at thepredetermined sub-carrier position in the symbol and decode theinformation described in the TMCC signal. The TMCC signal typicallycontains the system identifying information of the televisionbroadcasting system, the count down information for switching the TMCCinformation, the starting flag for broadcasting an emergency warningsignal, the segment identifying flag, the carrier modulation system, theconvolutional coding ratio and the time-directional interleavingpattern. The TMCC decoder 37 feeds the control section 38 with thedecoded various pieces of information.

[0136] The TMCC decoder 37 also detects the sync code (synchronous code)of the TMCC signal to generate a frame synchronizing signal. The framesynchronizing signal specifies the frame period of a frame and theleading position of the frame of the received OFDM signal that become ONat a predetermined position (e.g., the head) of the OFDM frame. The TMCCdecoder 37 generates the frame synchronizing signal typically byapplying a PPL for synchronized clock regeneration on the basis of thesync code of the TMCC signal. The frame synchronizing signal is fed tothe equalizer 32, the error correction circuit 36 and the controlsection 38 and used for controlling the synchronized timing and thepunctured switch timing of the SP signal.

[0137] The control section 38 controls the components and the overalloperation of the OFDM receiver. The control section 38 receives thepieces of information decoded by the TMCC decoder 37 as input and usesthem for controlling the components of the OFDM receiver and selectingparameters for it. The control section 38 additionally reads theinformation stored in the memory 39 and uses it for controlling thecomponents of the OFDM receiver and selecting parameters for it.

[0138] For each information channel to be used for receiving broadcastinformation contents, the RF frequency of the information channel, theguard interval length of the OFDM signal of the information channel, thetime-directional interleaving pattern, the carrier modulation system andthe convolutional coding ratio that are described in the TMCC signal arepreset in the memory 39. Additionally, the initial value of the fc errorcorrection signal fed to the fc correction circuit 27, the correctionvalue with the accuracy level of the gap of carrier waves to be outputfrom the WAFC circuit 36 and the initial value of the clock frequency ofthe sampling clock to be fed to the A/D converter 25 are also preset inthe memory 39.

[0139] The remote controller 40 is used by the user to select theinformation channel providing the program to be viewed and listened toby the user and the information on the selection is transmitted to thecontrol section 38 typically by infrared communication. The user mayselect the information channel by referring to the program guide of anewspaper or by referring to the EPG (Electric Program Guide) displayedon the monitor screen.

[0140] Now, the operation of starting the reception of signals when thepower supply is turned on, when a channel other than those of connectedtransmission is selected and when the selected channel of connectedtransmission is switched to another will be discussed below.

[0141] The operation of starting the reception of signals when the powersupply is turned on proceeds in a manner as described below.

[0142] Firstly, the user turns on the power supply and then selects aninformation channel by means of the remote controller 40. Then, theinformation specifying the selected information channel is fed to thecontrol section 38 as user selection information.

[0143] The control section 38 reads the RF frequency, the guard intervallength, the interleaving pattern, the carrier modulation system, theconvolutional coding ratio, the initial value of the fc error correctionsignal and the sampling frequency of the information channel selected bythe user from the memory 39. Then, the control section 38 selects thelocal oscillation frequency of the tuner 23, the guard interval lengthof FAFC•W-Sync 9, the interleaving pattern of the time-directionalde-interleaver 34, the convolutional coding ratio of the errorcorrection circuit 36, the initial value of the fc error correctionsignal to be fed to the fc correction circuit 27 and the initial valueof the clock frequency of the sampling clock to be fed to the A/Dconverter 25 on the basis of the information read out from the memory 39in order to start the reception of signals through the selectedinformation channel.

[0144] As the above values are selected by the control section 38, theOFDM receiver starts receiving signals.

[0145] Thus, with the OFDM receiver 20, the information channel to beused for receiving broadcast information contents, the RF frequency ofthe information channel, the guard interval length of the OFDM signalbroadcast through the information channel, the contents of the TMCCsignal added to the OFDM signal of the information channel (e.g., thetime-directional interleaving pattern, the carrier modulation system andthe convolutional coding ratio) the value of the clock frequency and theinitiative value of the fc error correction signal are preset in thememory 39. Therefore, as the user selects the information channel forreceiving signals, various values are selected according to theinformation preset in the memory 39.

[0146] If the guard interval information stored in the memory 39 iswrong and the stored value differs from the guard interval length of theOFDM signal that is actually received so that the signal is notcorrectly demodulated (and hence, for example, the TMCC signal is notdetected), it may be so arranged that the guard interval length issearched and selected for another time.

[0147] The operation of starting the reception of signals when a channelother then those of connected transmission is selected proceeds in amanner as described below.

[0148] Firstly, if the user who is receiving signals through aninformation channel wants to switch to some other information channel,he or she selects the desired information channel by means of the remotecontroller 40. Then, the information specifying the selected informationchannel is fed to the control section 38 as user selection information.

[0149] The control section 38 reads the RF frequency, the guard intervallength, the interleaving pattern, the carrier modulation system and theconvolutional coding ratio of the information channel selected by theuser from the memory 39. Then, the control section 38 selects the localoscillation frequency of the tuner 23, the guard interval length ofFAFC•W-Sync 9, the interleaving pattern of the time-directionalde-interleaver 34 and the convolutional coding ratio of the errorcorrection circuit 36 on the basis of the information read out from thememory 39 in order to start the reception of signals through theselected information channel.

[0150] Additionally, the control section 38 holds the sampling clockfrequency and the value of the fc error correction signal fed to the fccorrection circuit 27 (the correction value with the accuracy level ofthe gap of carrier waves as output from the WAFC circuit 36 and thecorrection value with the accuracy level of less than the gap of carrierwave as output from the FAFC circuit 29) to the respective valuesselected before the switch to the newly selected channel.

[0151] As the above values are selected by the control section 38, theOFDM receiver starts receiving signals.

[0152] Thus, with the OFDM receiver 20, the clock frequency and thevalue of the fc error correction signal are held to the respectivevalues selected before the switch to the new channel to start thereception of signal through the new channel. With this arrangement, itis possible to reduce the pull-in time for clock synchronization and forcarrier frequency synchronization.

[0153] The operation of starting the reception of signals when theselected channel of connected transmission is switched to anotherproceeds in a manner as described below.

[0154] Firstly, if the user who is receiving signals through aninformation channel of connected transmission wants to switch to someother information channel also of connected transmission, he or sheselects the desired information channel by means of the remotecontroller 40. Then, the information specifying the selected informationchannel is fed to the control section 38 as user selection information.

[0155] The control section 38 reads the RF frequency, the interleavingpattern, the carrier modulation system and the convolutional codingratio of the information channel selected by the user from the memory39. Then, the control section 38 selects the local oscillation frequencyof the tuner 23, the guard interval length of FAFC•W-Sync 9, theinterleaving pattern of the time-directional de-interleaver 34 and theconvolutional coding ratio of the error correction circuit 36 on thebasis of the information read out from the memory 39 in order to startthe reception of signals through the selected information channel.

[0156] Additionally, the control section 38 holds the position of theFFT window, the sampling clock frequency of the A/D converter 35 and thevalue of the fc error correction signal fed to the fc correction circuit27 (the correction value with the accuracy level of the gap of carrierwaves as output from the WAFC circuit 36 and the correction value withthe accuracy level of less than the gap of carrier wave as output fromthe FAFC circuit 29) to the respective values selected before the switchto the newly selected channel. Since the channels are those of connectedtransmission, the guard interval length remains unchanged before andafter the switch to the newly selected channel. Thus, the FFT window canbe held to the synchronized state if its position is unchanged so thatthe pull-in time required for synchronizing the FFT window can bereduced.

[0157] Still additionally, the frame synchronization is held unchangedif the information channel of connected transmission is switched toanother information channel also of connected transmission. According tothe invention, as shown in FIG. 14, the frame configuration of eachinformation channel is so designed for connected transmission that thetiming of transmission of each frame is synchronized with that ofanother. In other words, the timing of frame synchronization agree amongthe information channels. Thus, when an information channel of connectedtransmission is switched to another also of connected transmission, thetiming of frame synchronization of the information channel selectedbefore the switch can be used for controlling the frame synchronizationof the newly selected information channel without problem.

[0158] As the frame synchronization is held unchanged if the informationchannel of connected transmission is switched to another informationchannel also of connected transmission, no additional pull-in operationis required for frame synchronization so that the operation ofreproducing audio and video signals and that of decoding data can bestarted quickly.

[0159] The operations of the synchronizing circuit for the abovedescribed three different types of reception are summarized in the tablebelow. operations of synchronizing circuit type of power selection of aselection of a synchronization supply channel other then channel also ofON those of connected connected transmission transmission FFT windposition reset reset maintained clock frequency loading maintainedmaintained initial values carrier wave loading loading of initialloading of initial frequency (carrier initial values or values or wavegap) values maintained maintained carrier wave reset maintainedmaintained frequency (intra- carrier wave gap) frame reset resetmaintained synchronization

[0160] The initial value of the fc error correction signal is selectedin such a way that the fc error to be output when the RF frequency of aninformation channel is received for the first time is estimated inadvance and the fc error can be cancelled at the time of the start ofthe signal reception. The pull-in time required for carrier frequencysynchronization can be reduced by selecting such an initial value. Ifthe frequency of the local oscillator of the tuner 23 is highlyaccurate, the fc error can practically remain unchanged if the channelshows a frequency shift to a certain extent. If such is the case, thepull-in time required for carrier frequency synchronization can bereduced by holding the value of the fc error correction signal unchangedbefore and after the switch of the information channel. However, it maybe appreciated that an initial value may have to be selected when the fcerror can vary remarkably at the time of switching the channel.

[0161] As described in detail above, with this embodiment of OFDMtransmitter 1, the OFDM signals of a plurality of frequency domains aremultiplexed in the sense of frequency and collectively subjected to anoperation of inverse Fourier transform for connected transmission so asto synchronize the transmission frames to generate OFDM signals of therespective frequency domains.

[0162] Additionally, with this embodiment of OFDM transmitter 20, as aresult of connected transmission, when an information channel ofconnected transmission is switched to another also of connectedtransmission, the synchronism of the transmission frames is maintainedafter the switch. In other words, the synchronism of the transmissionframes is discontinued for the switch of the information channel.

[0163] As a result, the switch time required for shifting theinformation to be used for signal reception can be reduced and theoperation of reproducing video and audio signals and outputting data canbe started very quickly.

[0164] While the number of connected channels is three in the abovedescription, it should be noted that any number of information channelscan be connected for the purpose of the invention.

[0165] When shifting the information channel being used for signalreception, it is necessary to determine if the newly selectedinformation channel is connected with the previously selectedinformation channel for transmission or not.

[0166] The group of information channels connected for transmission maybe predefined as a system so that it can be determined with ease if thenewly selected information channel is connected with the previouslyselected information channel for transmission or not. However, with suchan arrangement, once the number of information connected fortransmission and the RF frequency are defined for the system before thestart of broadcasting, they can no longer modified and hence the systemcannot be operated flexibly to accept one or more than one additionalbroadcasting stations. In view of this problem, it is thereforedesirable to determine if the newly selected information channel isconnected with the previously selected information channel fortransmission or not on the basis of the information contained in thesignal transmitted through the information channel that is selectedbefore the switch.

[0167] Therefore, according to the invention, the information onconnected transmission on which if the previously selected informationchannel and the newly selected information channel are connected fortransmission or not is described in the TMCC signal or in the NIT(network information table) defined in the MPEG-2 Systems.

[0168] Now, examples of such description will be discussed below.

[0169] Firstly, an example of description in the TMCC signal will bedescribed.

[0170] The ISDB-Tn Standard applicable to terrestrial digital audiotransmission in Japan assigns the 188 MHz through 194 MHz band and the192 MHz through 198 MHz band (with a bandwidth of 6 MHz) to radiobroadcasting. Additionally, according to the ISDB-Tn Standard, up to 13segments (each segment corresponding to an information channel) can bemultiplexed for connected transmission within the bandwidth of 6 MHz.

[0171] For connected transmission, each of the channel encoders 2generates an OFDM signal of the frequency domain assigned to thecorresponding information channel for transmission and the generatedOFDM signals are subjected to frequency conversion and multiplexing sothat they may be transmitted collectively.

[0172] When generating the frame configuration for an OFDM signal, thecorresponding on of the channel encoders 2 contains the number ofconnected segments (B₁₁₀ through B₁₁₃) and the segment number of thesignal to be transmitted (B₁₁₄ through B₁₁₇) in B₁₁₀ through B₁₁₇ in theTMCC information (102 bits).

[0173] The number of connected segments is the total number of segmentsconnected for transmission with the signal to be transmitted (thatcontains the TMCC signal). The number of connected segments is threewhen three information channels are connected for transmission andthirteen when thirteen information channels are connected fortransmission. FIG. 18 shows an example of description on the number ofconnected segments. If two segments are connected, “0010” is describedin the bits of (B₁₁₀ through B₁₁₃) and, if three segments are connected,“0011” is described in the bits of (B₁₁₀ through B₁₁₃). Similarly, iffour segments are connected, “0100” is described in the bits of (B₁₁₀through B₁₁₃) and the described value is incremented as another segmentis added so that, if twelve segments are connected, “1100” is describedin the bits of (B₁₁₀ through B₁₁₃) and, if thirteen segments areconnected, “1101” is described in the bits of (B₁₁₀ through B₁₁₃). If nosegment is connected for transmission (and hence the segment is usedindependently for transmission), “1111” is described in the bits of(B₁₁₀ through B₁₁₃). The numerical values that are not assigned refersto a reserved domain. Note that the so-called 3 segments format definingthe hierarchical structure is not included in connected transmission anddescribed as independent transmission.

[0174] The segment number shows the information on the relative positionof the signal to be transmitted (that contains the TMCC signal) in theconnected transmission.

[0175] According to the ISDB-Tn Standard, segment #0 is assigned to thecenter segment being used for connected transmission and the segmentnumber is incremented alternately to the left and to the right from thecenter segment as shown in FIG. 19. Thus, when thirteen segments areconnected for transmission, segment #0 through #12 are assigned to therespective segments in a manner as shown in FIG. 19. Similarly, whenthree segments are connected for transmission, segment #0 through #2 areassigned to the respective segments in a manner as shown in FIG. 20 and,when six segments are connected for transmission, segment #0 through 5are assigned to the respective segments in a manner as shown in FIG. 21.

[0176]FIG. 22 schematically illustrates an example of description on thesegment numbers in the TMCC signal.

[0177] Referring to FIG. 22, “1111” is described when the signal to betransmitted (that contains the TMCC signal) is located at the segmentposition with segment #0 and “1110” is described when the signal to betransmitted is located at the segment position with segment #1, while“1101” is described when the signal to be transmitted is located at thesegment position with segment #2. In this way, the described value isdecremented by one as the segment position is shifted by one so that“0011” is described when the signal to be transmitted is located at thesegment position with segment #12. The domains where no segment numberis assigned are reserved domains.

[0178] Then, upon obtaining the TMCC information, the receiver 21determines if the information selected before the switch is connected tothe information channel after the switch for transmission.

[0179] When an instruction for switching the information channel isgiven by the user, the RF frequency of the newly selected informationchannel is input by the user (or the information on the newly selectedprogram or broadcast station is input by the user and the information onthe selection is interpreted to obtain the RF frequency of the selectedinformation channel typically by referring to a table). Subsequently,the difference between the RF frequency of the current informationchannel and that of the newly selected information channel iscalculated. Then, the frequency difference is divided by the bandwidthof an information channel (and hence of a segment: 430 kHz) to reducethe frequency difference into the difference in the corresponding numberof segments.

[0180] Thereafter, it is determined if the newly selected informationchannel is located inside or outside the information channels connectedfor transmission on the basis of the obtained number of segments, thenumber of connected segments as described in the TMCC information andthe segment number. For instance, as shown in FIG. 23, if the TMCCsignal of the transmitted signal before the switch of the informationchannel shows that there are eight connected segments and the segmentnumber is #4. Then, if the difference of segments separating the currentinformation channel and the newly selected information channel is −5 asdetermined by the reducing operation, the segment number of the newlyselected information channel is #5 and hence the newly selectedinformation channel is found in the information channels connected fortransmission. Contrary, if the difference of segments separating thecurrent information channel and the newly selected information channelis +4 as determined by the reducing operation, no segment number isassigned to the newly selected information channel and hence the newlyselected information channel is found outside the information channelsconnected for transmission.

[0181] Therefore, by describing the number of connected segments andtheir respective segment numbers in the TMCC information, it is possibleto easily determine if the newly selected information channel is foundwithin the connected information channels or outside thereof.

[0182] Now, an example of description of an NIT as defined in MPEG-2Systems will be described below.

[0183] As pointed out earlier, according to the ISDB-Tn Standard, it ispossible to multiplex up to thirteen segments (each segmentcorresponding to an information channel) for connected transmissionwithin a bandwidth of 6 MHz. When describing the information onconnected transmission in the NIT, the information channels within thebandwidth of 6 MHz are divided into groups on the basis of the unit ofthe information channels connected for transmission. Then, a uniquegroup ID is assigned to each of the groups and described in the NIT.

[0184] For instance, assume that group ID0# is assigned to the group forconnected transmission with the lowest frequency band within thebandwidth of 6 MHz, group ID1# is assigned to the next group forconnected transmission and, in this way, group IDs are assigned to atotal of seven groups for connected transmission so that group ID6# isassigned to the seventh group for connected transmission. Then, theinformation on connected transmission is described in the NIT byexpressing each of the group numbers in three bits as shown in FIG. 24.Some other unique value (e.g., “111”) will be assigned for aninformation channel to be used for independent transmission (withoutbeing connected for transmission).

[0185] Assume that five connected transmission groups are found in abandwidth of 6 MHz, each group containing two segments that areconnected from transmission, along with a single segment for independenttransmission. Then, group IDs as shown in FIG. 25 may be assigned to theconnected transmission groups.

[0186] In such an NIT, a same and identical description will be made bythe source encoder 1 a for all the information channels. Then, at thereceiver side, the NIT is analysed by the MPEG decoder 21 as shown inFIG. 15 and the obtained information is fed back to the receiver side.In other words, given an information channel, the receiver 21 can alwaysdetermine the connected transmission group to which the informationchannel belongs by analysing the NIT.

[0187] Thus, upon receiving the information on connected transmission asdescribed in the NIT, the receiver 21 determines if the newly selectedinformation channel is connected to the previously selected informationchannel for transmission or not in a manner as described below.

[0188] Firstly, upon receiving an instruction for switching theinformation channel, it is determined if the newly selected informationchannel is found within the frequency channel that can be used forconnected transmission (or the bandwidth of 6 MHz as described above. Inthe case of ISDB-Tn, no connected transmission is realized with anyinformation channel outside the bandwidth). If the newly selectedinformation channel is found outside the bandwidth of 6 MHz, it isdetermined that the information channel is not connected fortransmission. If, on the other hand, the newly selected informationchannel is found within the bandwidth of 6 MHz, the NIT is referred toin order to compare the connected transmission group ID of thepreviously selected information channel and that of the newly selectedinformation channel. If the two group IDs are found to agree with eachother as a result of the comparison, the newly selected informationchannel is determined to be also connected for transmission. If, on theother hand, the two group IDs do not agree with each other, it isdetermined that the newly selected information channel is out ofconnected transmission.

[0189] In this way, it can be determined with ease if the newly selectedinformation channel is also connected for transmission or out ofconnected transmission by describing the connected transmission groupIDs in the NIT.

[0190] The information on connected transmission may be contained eitherin the TMCC information or in the NIT. Alternatively it may be containedin both the TMCC information and the NIT.

[0191] After determining that the newly selected information channel isalso connected for transmission or out of connected transmission, theframe synchronism is maintained if the arrangement of OFDM frames issynchronized at the time of transmission. However, only the synchronismof the FFT windows may be maintained if the arrangement of OFDM framesis not synchronized.

What is claimed is:
 1. An OFDM receiving device for receiving anorthogonally frequency divided and multiplexed signals comprising: areceiver for selecting an information channel to be received andreceiving the RF signal of the selected information channel; anorthogonal demodulator for orthogonally demodulating the signal receivedby said receiver and outputting an OFDM signal of a base band; a Fouriertransform section for performing an operation of Fourier transform onsaid OFDM signal of the base band and outputting an OFDM signal of afrequency domain; a decoder for decoding said OFDM signal of thefrequency domain; and a frame synchronism control section for detectingthe synchronism of transmission frames of said channel data andcontrolling the synchronism of said decoder; said frame synchronismcontrol section being adapted to maintain the synchronism of thetransmission frames for switching the information channel to be used forsignal reception among the channels connected for transmission in thecase of receiving OFDM signals of frequency domains of a plurality ofinformation channels multiplexed in a direction of frequency andcollectively subjected to an operation of inverse Fourier transform forconnected transmission.
 2. An OFDM receiving device for receiving anorthogonally frequency divided and multiplexed comprising: an OFDMreceiving device comprising a receiver for selecting an informationchannel to be received and receiving an RF signal of the selectedinformation channel; an orthogonal demodulator for orthogonallydemodulating the signal received by said receiver and outputting theOFDM signal of a base band; a Fourier transform section for performingan operation of Fourier transform on said OFDM signal of the base bandand outputting an OFDM signal of a frequency domain; a decoder fordecoding said OFDM signal of the frequency domain; a control section forcontrolling said Fourier transform section and said decoder; and saidOFDM signal of the base band containing information on connectedtransmission indicating whether the OFDM signal to be transmitted to aninformation channel and the OFDM signal to be transmitted to otherinformation channels are transmitted in a connected state bymultiplexing the OFDM signals of the frequency domain of a plurality ofthe information channel in a direction of the frequency and performingan inverse Fourier transform on the multiplexed OFDM signalscollectively, said control section being adapted to determine whetherthe information channel being used for signal reception is coupled tothe information channel to be selected for signal reception by switchingby referring to said information on connected transmission.
 3. The OFDMreceiving device according to claim 2, wherein said control sectionmaintains the synchronism of the transmission frames when the newlyselected information channel is connected with the previously selectedinformation channel.
 4. An OFDM receiving method for receivingorthogonally frequency divided and multiplexed signals comprising a stepof maintaining the synchronism of the transmission frames for switchingthe information channel to be used for signal reception among thechannels connected for transmission in the case of receiving OFDMsignals of frequency domains of a plurality of information channelsmultiplexed in a direction of frequency and collectively subjected to anoperation of inverse Fourier transform for connected transmission.
 5. AnOFDM receiving method for receiving orthogonally frequency divided andmultiplexed signals comprising the steps of; receiving the OFDM signalof the base band containing information on connected transmissionindicating whether the OFDM signal to be transmitted to an informationchannel and the OFDM signal to be transmitted to other informationchannels are transmitted in a connected state by multiplexing the OFDMsignals of the frequency domain of a plurality of the informationchannels in a direction of the frequency and performing an inverseFourier transform on the multiplexed OFDM signals collectively; and ofdetermining whether the information channel being used for signalreception is coupled to the information channel to be selected forsignal reception by switching being determined by referring to theinformation on the connected transmission.
 6. The OFDM receiving methodaccording to claim 5, wherein the synchronism of the transmission framesis maintained when the selected information channel is connected withthe previously selected information channel.